Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

web_src/js/modules/stores.ts

@@ -0,0 +1,10 @@
+import {reactive} from 'vue';
+
+let diffTreeStoreReactive;
+export function diffTreeStore() {
+  if (!diffTreeStoreReactive) {
+    diffTreeStoreReactive = reactive(window.config.pageData.diffFileInfo);
+    window.config.pageData.diffFileInfo = diffTreeStoreReactive;
+  }
+  return diffTreeStoreReactive;
+}